Pendulum Construction

 Some time ago I published a couple of posts about the clocks Pendulum, firstly a Simple Pendulum and then another post about the more complex Compound pendulum.
This time however it's time to look at the construction of the Pendulum and how it interacts with the rest of the clock. The following are all based on the construction of the simple Pendulum but they can also be read as applying to the Compound pendulum as well.

The view above here shows the back of the Clock with the pendulum pivot fitted to the same shaft as the Escapement Arm at the top, and at the bottom, the pendulum Bob is attached. It is these two extremes that I shall concentrate on through the rest of this article but just a word concerning the Pendulum Rod itself.

I always like to use Carbon Fibre tubing for its stiffness and lightness but other materials such as wooden dowels or Aluminium tubing can work just as well.

Now looking at the top of the pendulum first I have shown two design routes that I have used in the past. The first on the left shows the conventional layout with the Pendulum pivot resting in a groove on the protruding stub, this is really the minimum friction approach as the sharp edges on both the Pivot and the groove give the minimal surface area of contact. To get the clock working the swinging Pendulum needs to be connected to the Escapement arm on the other side of the Back frame to do this the Yoke is used which lightly grips either side of the pendulum rod carrying the Yoke and the Escapement Arm with it. This works quite well but it does generate some friction as the Yoke slides up and down the Pendulum Rod as it swings because the two items are not hanging from the same pivots, the Yoke pivoting a couple of inches below the Pendulum itself.
However depending on the materials used this will end up wearing quite quickly so some harder material than wood should ideally be used, for instance, Brass works quite well. 

As we are not aiming for the ultimate accuracy in the clock, there is another route shown on the right which is a little easier to build and avoids the friction losses between the Yolk and the Pendulum rod. in this case the Pendulum head is connected directly to the Escapement Arm by having both swings on the same pivot and using the Connector pin fixed directly into both parts through a slot in the Back Frame. This is the method I have chosen in many of my later clock designs.

Moving to the Bottom of the Pendulum there are quite a few variations on the construction that I have developed over the years. The first of these shown here to the left is the use of an adjusting nut. You can see that at the very bottom of the wooden rod the material has changed to Brass and a threaded section added to the end. The Pendulum Bob is slid onto the bottom of the pendulum and then followed by an adjusting nut, the effective length of the pendulum can now be adjusted by turning the nut so that it moves upward to make the clock run quicker or downwards to make it slower. The reasoning behind these adjustments is discussed in the earlier article on the Simple Pendulum. The use of the adjusting nut makes it a lot easier to accurately adjust the rate of the clock because very small movements can be accurately made to the effective length of the pendulum. The disadvantage is that the construction is more difficult because of the joining of the two parts of the Rod together although drilling the ends of the parts and pinning them together can make it easier.

I have not mentioned the shape of the pendulum Bob but to all intents and purposes it doesn't matter as you can have any shape you like. I generally either use a Brass rod cut to a length, the same as in the illustration above showing the back of the clock. Alternatly a disc shape like shown above left.

Moving on, the first of the alternative methods of constructing the Pendulum Bob is shown below. This method requires that a rectangular slot is cut through the centre of the Bob and then a Lock component fitted into it. This Lock component has a hole at its inner end that is slightly offset from the centre of the Pendulum Rod passing through it. The outside end of the Lock is shaped like a bow, so by pressing in the centre of the bow the hole in the other end can be aligned with the Pendulum Rod to allow it to pass through. When pressure is released from the bow the hole is pulled against the rod locking it in place. In this way, the Bob can be slid up or down by pressing the bow as you move the Bob. Adjustment is not so accurate as with the threaded nut but it is easier to construct and gives a lot more adjustment movement. The weights added to this Pendulum Bob design give the Pendulum a little more momentum to overcome any small glitches in the gear train driving the movement of the clock.

Another version of this concept is shown below it has the Lock moving from the side and the actual locking is made using a small metal spring. The advantage of this is that adjustments can be made more easily without putting your hand behind the pendulum.

A completely different approach shown next adds a little fun, I used this on a Clock built for my Great Grand son and has a face built up from the 3 layers of the pendulum, the centre layer containing a couple of spring elements shown in blue, they press on the sides of the Pendulum Rod to provide the friction to hold it in place, adjustment is therefore quite easy simply sliding the Bob up or down. Again not as accurate as the screw method but simpler and visually far more iinteresting.

Another way that we can use to add more interest to the rather bland appearance of the wooden disc is to Laser engrave or 'Vee carve' a pattern into the wood itself. Three examples of what is possible are shown here. By 'Vee carve' I mean to use a vee shaped bit for CNC machining where the cutter uses the 3rd dimensions for producing the thick and thin sections of the profile being cut. If your CAM software does not have this feature you could try 'Carbide Create' it is available as a free download for either Windows or Mac.

Whether you laser engrave or V carve you will need a pattern to start with so I have attached some example DXF files for you to try.